The invention relates to an actuator for electromagnetically controlling a valve with a device for determining the position of the retaining plate. This device is made up of a magnetic field sensor and a magnetic field transmitter. Actuators of this kind for electromagnetically controlling a valve are known, for example, from U.S. Pat. No. 4,957,074.
An actuator for electromagnetically controlling a valve essentially comprises the closing magnet and the opening magnet which are separated from each other by at least one component of non-ferromagnetic material. This component can be in the form of a housing part, for example. Between the closing magnet and the opening magnet there is a retaining plate made of a ferromagnetic material that is moved when current flows through the operating coil of the opening magnet or the operating coil of the closing magnet in the respective direction. The yoke of the opening magnet has a bushing for a plunger which transfers the forces acting on the retaining plate to at least one gas change valve.
The actuator can be designed in such a way that, for example, the actuator spring is arranged on the side of the actuator that is opposite to the gas change valve and on the outside of the closing magnet. In the extension of the plunger, there is provided for this purpose a pushrod incorporating an actuator spring plate and mounted through a bushing in the yoke of the closing magnet. The yoke of the closing magnet has a formed shape that creates a wall around the bushing of the pushrod and in which an internal thread is provided. A screw cap is screwed into the internal thread of the wall forming, together with the wall, a hollow space in which the actuator spring that rests on the actuator spring plate is arranged. By turning the screw cap, the pretensioning of the actuator spring can be changed so that the rest position of the retaining plate can be adjusted.
An actuator and a gas change valve form a functional unit, where the gas change valve, as in a conventional cylinder head with camshafts, is pulled into the valve seat of the cylinder head by means of a valve spring and a valve spring plate.
When a functional unit comprising an actuator and a gas change valve has been mounted on the internal combustion engine, the valve stem of the gas change valve, the plunger and the pushrod of the actuator are pressed against each other. In the rest position of the functional unit, the retaining plate is precisely in the middle between the opening magnet and the closing magnet, the valve spring and the actuator spring being pretensioned. The valve plate of the gas change valve is in the mid-position between the valve seat of the cylinder head, at which the gas change valve is closed, and the position in which the gas change valve is fully open.
When an actuator is used to electromagnetically control a valve, it is of fundamental importance that the opening magnet and the closing magnet be operated at exactly the correct time with a precisely measured current. To provide these parameters, each actuator has a device for determining the position of the retaining plate that supplies to the control electronics assigned to the actuators a signal that is proportional to the position of the retaining plate. The device comprises, for example, a stationary magnetic field sensor attached to the yoke of the closing magnet and a magnetic field transmitter fitted to the oscillating pushrod. The control electronics calculates the current required to control the motion characteristics of the retaining plate on the basis of the magnetic field sensor signal, and also the times at which the opening magnet and the closing magnet are switched.
The magnetic field sensors sense the field strength of a magnetic field transmitter fitted to one of the oscillating parts of the actuator. A device of this kind for determining position is known from U.S. Pat. No. 4,957,074 named at the outset.
The disadvantage of this device for determining the position of the retaining plate is that the field line curve of the magnetic field transmitter is affected by the metallic parts of the actuator, some of which oscillate, such as for example the actuator spring.